1. Field of the Invention
The invention pertains to the rapid acquisition of composite code signals and particularly to the secure acquisition thereof. The invention is particularly adaptable for use in spread spectrum communication systems.
2. Description of the Prior Art
Composite codes have been utilized, for example, in systems for obtaining the range of an object such as a space vehicle with respect to a base transmitter or for providing a data communication function in systems, for example, of the spread spectrum transmission type. Such ranging systems are described in the IEEE Transactions on Aerospace and Electronic Systems, Volume AES-3, No. 1, January 1967, "Designing Pseudo-Random Coded Ranging Systems" by John H. Painter, page 14 as well as in the JPL Research Summary, No. 36-10, "Commmunications Research" by M. F. Easterling et al., page 26.
Composite codes for use in such systems may be composed from component codes in accordance with a variety of composition rules. For example, the component codes may be binary state codes which are combined in accordance with a Boolean majority voting rule whereby a bit in the composite code is binary ZERO when half or more of the corresponding bits of the component codes are ZERO, and is binary ONE if more than half of the corresponding bits of the component codes are ONE.
As is well known, composite codes have been utilized in systems of the type described for rapid signal acquisition where acquisition is understood to mean the alignment of a reference code with the transmitted acquisition code until the peak of the correlation function therebetween is attained, as explained in the above-cited references. The correlation signal may then be utilized to adjust the system timing to track the acquisition code thereby remaining locked thereto.
The acquisition composite code may be rapidly acquired by first aligning one of the component codes with the received signal. The alignment is achieved by continuously adjusting the phase of the one component code until the peak of the correlation function between the component code and the received signal is attained. Since all of the component codes are maintained in bit synchronism and are simultaneously swept in-phase with each other during the alignment of the first component code, all of the component codes are positioned in bit alignment with the received code, that is, with the boundaries between bits in each code occurring in synchronism. If the acquisition code is of a type whereby all of the component codes correlate at least partially therewith, the remaining component codes may be aligned with the acquisition code by stepping each code sequence one bit position at a time past the received code until the respective correlation peaks are attained. This particular rapid acquisition procedure is well known and is described in detail in the above-cited references.
Two known composite codes suitable for rapid acquisition are the majority voting code described above and a three component code composed by forming the modulo-2 sum of one of the component codes with the logical AND combination, or logical product, of the remaining two codes. These codes are suitable for rapid acquisition in both ranging and data communication systems. The component and composite codes utilized may, for example, be pseudo-random codes.
A problem existent in such ranging systems is the false acquisition of a signal by accidental mistiming with respect to one or more of the component codes or by a deliberate analytical jamming or spoofing by an enemy device transmitting one of the component codes at a significantly higher signal strength than that of the corresponding component in the desired signal. The ranging system would thus lock onto the enemy transmitter providing false and misleading range information.
In data communication systems, particularly of the covert variety, where the acquisition composite code signal may be modulated by data, the system may be subject to accidental mistiming and deliberate jamming or spoofing as discussed above thus preventing the transmission of vital data or causing the receipt of false information transmitted by the enemy device.
A further disadvantage of these prior systems is that they may be subject to repeat jamming. Repeat jamming is the immediate retransmission, by an enemy device, of the signal to be acquired but at a substantially higher signal strength than that of the friendly transmission. The friendly receiver may lock onto the enemy transmitter by reason of sidelobes of the correlation function between the composite codes utilized.